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Changes in Avian Migration Timing: Phenotypic Plasticity or Microevolution?
Van Buskirk, J., Mulvihill, R.S. and Leberman, R.C. 2012. Phenotypic plasticity alone cannot explain climate-induced change in avian migration timing. Ecology and Evolution 2: 2430-2437.

The authors write that "phenological responses to climate change are well-documented in plants and animals," and that "it is widely assumed that much of the shift in phenology is due to facultative changes in the activities or physiologies of individuals induced by environmental conditions, known as phenotypic plasticity." However, they go on to say that "phenotypic plasticity is not the only mechanism that can produce population responses to climate change," noting that "gradual or sudden shifts in the selection regime can be triggered by environmental change, and these in turn can alter the genetic composition of populations." In fact, they indicate that "rapid evolved responses to climate change are widely anticipated by evolutionary biologists (Bradshaw and Holzapfel, 2001; Davis et al., 2005; Gienapp et al., 2008; Hoffman and Willi, 2008; Hoffmann and Sgro, 2011), and already have been observed in a few cases (Umina et al., 2005; Bradshaw and Holzapfel, 2008)."

What was done
In an amazing endeavor to explore this subject in some depth, between June 1961 and August 2006, Van Buskirk et al. operated some 35 mist nets for 5-6 days a week on a 10-ha study area on the Powdermill Nature Reserve in Pennsylvania (USA), in order to obtain the data needed to estimate the magnitude of temperature-induced phenotypic plasticity in spring arrival date of 27 species of birds in eastern North America, by recording the effects of annual variation in spring temperature on the behavior of thousands of individuals. This they did with the thought in mind that if phenotypic plasticity could not account for the totality of observed shifts in migration phenology over this time period, what remained unaccounted for could be attributed, at least partly, to microevolutionary change.

What was learned
The three researchers report that "for 2441 individuals detected in multiple years, arrival occurred earlier during warm years, especially in species that migrate short distances," but they say that the change in phenology predicted "under a model of pure phenotypic plasticity" fell far short of the change in arrival date they observed, explaining only 13-25% of the climate-induced trend in phenology. Although acknowledging that their approach to the problem "probably underestimates the full scope of plasticity," Van Buskirk et al. say their data suggest that "part of the response to environmental change [warming] has been caused by microevolution." And they add that "the estimated evolutionary rates [0.016 haldanes] are plausible."

What it means
In discussing the significance of their findings, the three scientists write that "rapid genetic response to climate change is widely seen as a critical component of the kind of adaptation that will be required of many organisms," especially in a world that warms as suggested by the models employed by the IPCC. And they state, in addition, that "the contribution of plasticity will allow individuals to adjust their phenotype to short-term environmental fluctuations, which are projected to increase under most scenarios of climate change."

Bradshaw, W.E. and Holzapfel, C.M. 2001. Genetic shift in photoperiodic response correlated with global warming. Proceedings of the National Academies of Science USA 98: 14,509-14,511.

Bradshaw, W.E. and Holzapfel, C.M. 2008. Genetic responses to rapid climate change: it's seasonal timing that matters. Molecular Ecology 17: 156-166.

Davis, M.B., Shaw, R.G. and Etterson, J.R. 2005. Evolutionary responses to changing climate. Ecology 86: 1704-1714.

Gienapp, P., Teplitsky, C., Alho, J.S., Mills, J.A. and Merila, J. 2008. Climate change and evolution: disentangling environmental and genetic responses. Molecular Ecology 17: 167-178.

Hoffmann, A.A. and Sgro, C.M. 2011. Climate change and evolutionary adaptation. Nature 470: 479-485.

Hoffmann, A.A. and Willi, Y. 2008. Detecting genetic responses to environmental change. Nature Reviews Genetics 9: 421-432.

Umina, P.A., Weeks, A.R., Kearney, M.R., McKechnie, S.W. and Hoffmann, A.A. 2005. Rapid shift in a classic clinal pattern in Drosophila reflecting climate change. Science 308: 691-693.

Reviewed 29 May 2013